The heart is the center of a person's circulatory system. It includes an electro-mechanical system performing two major pumping functions. The left side of the heart, including the left atrium (LA) and the left ventricle (LV), draws oxygenated blood from the lungs and pumps it to the organs of the body to supply their metabolic needs for oxygen. The right side of the heart, including the right atrium (RA) and the right ventricle (RV), draws deoxygenated blood from the body organs and pumps it to the lungs where the blood gets oxygenated. These pumping functions result from contractions of the myocardium (cardiac muscles). In a normal heart, the sinoatrial (SA) node, the heart's natural pacemaker, generates electrical impulses, called action potentials, that propagate through an electrical conduction system to various regions of the heart and excite the myocardial tissues of these regions. Coordinated delays in the propagations of the action potentials in a normal electrical conduction system cause the various portions of the heart to contract in synchrony and result in efficient pumping function.
A blocked or otherwise damaged electrical conduction system causes irregular contractions of the myocardium, a condition generally known as arrhythmia. Arrhythmia reduces the heart's pumping efficiency and hence diminishes the blood flow to the body. A deteriorated myocardium has decreased contractility, also resulting in diminished blood flow. A heart failure patient usually suffers from both a damaged electrical conduction system and a deteriorated myocardium. Cardiac pacing therapy has been applied to treat arrhythmia and heart failure. For example, cardiac resynchronization therapy (CRT) applies left ventricular or biventricular pacing to restore synchronized contractions. A CRT system may include electrodes placed in the RA, the RV, and the LV to deliver pacing pulses to one or more of these heart chambers for restoring cardiac synchrony by artificially coordinating atrioventricular and/or interventricular myocardial activation delays.